Laser ablation is a method to clean or refresh surfaces. Contaminants or surface layers are ablated (destroyed) by applying laser energy directly to the surface. The laser is tuned to ablate the contaminants or surface layers while leaving the underlying surface material intact. The effectiveness of laser ablation is substantially due to the laser energy absorbed at the surface. If too much energy is absorbed, the surface may be damaged (over ablated). If too little energy is absorbed, the contaminants or surface layers may not be sufficiently affected (under ablated).
In conventional laser ablation, the laser beam is scanned across the surface in a raster fashion. The laser beam is scanned quickly in one direction (the scan direction) and relatively slowly in another direction (the traverse direction). The laser beam generally is scanned so quickly in the scan direction that the laser appears, and may be treated, as a laser sheet. Generally, laser ablation is performed by scanning a rectangular area with the width and breadth corresponding to the scan and traverse directions. The laser sheet in the scan direction is swept in the traverse direction at a traverse scan speed to cover the area to be ablated.
The effectiveness of laser ablation is determined by the irradiance (surface density of laser power) and fluence (surface density of accumulated laser energy), among other factors such as laser wavelength, surface material, and material to be ablated. For conventional laser ablation, the laser power is constant during ablation and the irradiance and fluence are substantially constant because the ablation area is rectangular with a constant scan width. If the scan width is changed, the irradiance and/or the fluence is changed. For example, a different scan width leads to a different width of the laser sheet and a different irradiance for the laser sheet (because the same laser power is spread over a different scan width). Hence, conventional laser ablation suffers from being restricted to rectangular areas (with a constant scan width), over ablation (due to exposing more area than necessary or exposing an area for too long), and/or under ablation (due to exposing an area for too little time).